The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for protecting a power conversion system based on at least a feedback signal. Merely by way of example, the invention has been applied to a flyback power conversion system. But it would be recognized that the invention has a much broader range of applicability.
Generally, a conventional power conversion system often uses a transformer to isolate the input voltage on the primary side and the output voltage on the secondary side. To regulate the output voltage, certain components, such as TL431 and an opto-coupler, can be used to transmit a feedback signal from the secondary side to a controller chip on the primary side. Alternatively, the output voltage on the secondary side can be imaged to the primary side, so the output voltage is controlled by directly adjusting some parameters on the primary side. Then, some components, such as TL431 and an opto-coupler, can be omitted to reduce the system costs.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system with primary-side sensing and regulation. The power conversion system 100 includes a primary winding 110, a secondary winding 112, an auxiliary winding 114, a power switch 120, a current sensing resistor 130, an equivalent resistor 140 for an output cable, resistors 150 and 152, and a rectifying diode 160. For example, the power switch 120 is a bipolar junction transistor. In another example, the power switch 120 is a MOS transistor.
To regulate the output voltage within a predetermined range, information related to the output voltage and the output loading often needs to be extracted. For example, when the power conversion system 100 operates in a discontinuous conduction mode (DCM), such information can be extracted through the auxiliary winding 114. When the power switch 120 is turned on, the energy is stored in the secondary winding 112. Then, when the power switch 120 is turned off, the stored energy is released to the output terminal during a demagnetization process. The voltage of the auxiliary winding 114 maps the output voltage on the secondary side as shown below.
                                                                        V                FB                            =                            ⁢                                                                    R                    2                                                                              R                      1                                        +                                          R                      2                                                                      ×                                  V                  aux                                                                                                        =                            ⁢                              k                ×                n                ×                                  (                                                            V                      o                                        +                                          V                      F                                        +                                                                  I                        o                                            ×                                              R                        eq                                                                              )                                                                                        (                  Equation          ⁢                                          ⁢          1                )            where VFB represents a voltage at a node 154, and Vaux represents the voltage of the auxiliary winding 114. R1 and R2 represent the resistance values of the resistors 150 and 152 respectively. Additionally, n represents a turns ratio between the auxiliary winding 114 and the secondary winding 112. Specifically, n is equal to the number of turns of the auxiliary winding 114 divided by the number of turns of the secondary winding 112. Vo and Io represent the output voltage and the output current respectively. Moreover, VF represents the forward voltage of the rectifying diode 160, and Req represents the resistance value of the equivalent resistor 140. Also, k represents a feedback coefficient as shown below:
                    k        =                              R            2                                              R              1                        +                          R              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
FIG. 2 is a simplified diagram showing a conventional operation mechanism for the flyback power conversion system 100. As shown in FIG. 2, the controller chip of the conversion system 100 uses a sample-and-hold mechanism. When the demagnetization process on the secondary side is almost completed and the current Isec of the secondary winding 112 almost becomes zero, the voltage Vaux of the auxiliary winding 114 is sampled at, for example, point A of FIG. 2. The sampled voltage value is usually held until the next voltage sampling is performed. Through a negative feedback loop, the sampled voltage value can become equal to a reference voltage Vref. Therefore,VFB=Vref  (Equation 3)
Combining Equations 1 and 3, the following can be obtained:
                              V          o                =                                            V              ref                                      k              ×              n                                -                      V            F                    -                                    I              o                        ×                          R              eq                                                          (                  Equation          ⁢                                          ⁢          4                )            Based on Equation 4, the output voltage decreases with the increasing output current.
Additionally, in the discontinuous conduction mode (DCM), the flyback power conversion system 100 can also regulate the output current regardless of the output voltage based on information associated with the waveform for the voltage Vaux of the auxiliary winding 114 as shown in FIG. 2.
For example, the output current is equal to an average of a secondary current 198 flowing through the secondary winding 112 during a switching period which includes a demagnetization period corresponding to the demagnetization process.
                              I          old                =                              1            2                    ⁢                      I            sec_pk                    ⁢                                    T              dem                                      T              s                                                          (                  Equation          ⁢                                          ⁢          5                )            where Iout represents the output current, Isec_pk represents the magnitude of the secondary current 198 when the switch 120 is turned off, Tdem represents the duration of the demagnetization period, and Ts represents the duration of the switching period.
As an example, according to Equation 5, the output current can be determined as follows:
                              I          out                =                              1            2                    ⁢          N          ⁢                      1            T                    ⁢                                    ∫              0              T                        ⁢                                                            V                  cs                                                  R                  s                                            ⁢                                                T                  dem                                                  T                  s                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          6                )            where N represents the turns ratio between the primary winding 110 and the secondary winding 112, Rs represents the resistance of the resistor 130, T represents an integration period, and Vcs represents a peak current-sensing signal associated with a primary current 196 flowing through the primary winding 110 in each switching cycle.
According to Equation 6, if Vcs and Tdem/Ts do not change much, the output current may be regulated regardless of the input voltage, the output voltage, or the inductance of the transformer including the primary winding 110 and the secondary winding 112, so the power conversion system 100 operates, for example, in a constant-current mode.
But when the power conversion system 100 operates in the constant-current mode, the power conversion system 100 needs to be protected. Hence it is highly desirable to improve the techniques of system protection.